That happens annoyingly often. It's not just bosses, it can also be high-level monsters. The map has a tendency to put long corridors on either side of the bridge so if you don't have ENDISWAL your chances of getting stuck like this are much higher.

I had it happen to me with Bleaty on the bridge a few days ago, which is not nice. Fortunately I was packing preps and had both reflex and quicksilver, so I was able to just barely get through him as a level 4 character.

Heh. In terms of accesability, this might be the smallest subdungeon I've ever found. Rather fitting for the theme, though.For anybody wondering, its the exploding signposts subdungeon. Luckily, I found Endiswall later on- but still, it was both a bit annoying and funny to me .